Most current wireless communication systems are composed of nodes configured with a single transmit and receive antenna. However, for a wide range of wireless communication systems, it has been predicted that the performance, including capacity, may be substantially improved through the use of multiple transmit and/or multiple receive antennas. Such configurations form the basis of many so-called “smart” antenna techniques. Such techniques, coupled with space-time signal processing, can be utilized both to combat the deleterious effects of multipath fading of a desired incoming signal and to suppress interfering signals. In this way both the performance and capacity of digital wireless systems in existence or being deployed (e.g., CDMA-based systems, TDMA-based systems, WLAN systems, and OFDM-based systems such as IEEE 802.11a/g) may be improved.
The impairments to the performance of wireless systems of the type described above may be at least partially ameliorated by using multi-element antenna systems designed to introduce a diversity gain and suppress interference within the signal reception process. This has been described, for example, in “The Impact of Antenna Diversity On the Capacity of Wireless Communication Systems”, by J. H. Winters et al, IEEE Transactions on Communications, vol. 42, No. 2/3/4, pages 1740-1751, February 1994. Such diversity gains improve system performance by mitigating multipath for more uniform coverage, increasing received signal-to-noise ratio (SNR) for greater range or reduced required transmit power, and providing more robustness against interference or permitting greater frequency reuse for higher capacity.
Within communication systems incorporating multi-antenna receivers, it is known that a set of M receive antennas are capable of nulling up to M−1 interferers. Accordingly, N signals may be simultaneously transmitted in the same bandwidth using N transmit antennas, with the transmitted signal then being separated into N respective signals by way of a set of N antennas deployed at the receiver. Systems of this type are generally referred to as multiple-input-multiple-output (MIMO) systems, and have been studied extensively. See, for example, “Optimum combining for indoor radio systems with multiple users,” by J. H. Winters, IEEE Transactions on Communications, Vol. COM-35, No. 11, November 1987; “Capacity of Multi-Antenna Array Systems In Indoor Wireless Environment” by C. Chuah et al, Proceedings of Globecom '98 Sydney, Australia, IEEE 1998, pages 1894-1899 November 1998; and “Fading Correlation and Its Effect on the Capacity of Multi-Element Antenna Systems” by D. Shiu et al, IEEE Transactions on Communications vol. 48, No. 3, pages 502-513 March 2000.
One aspect of the attractiveness of multi-element antenna arrangements, particularly MIMOs, resides in the significant system capacity enhancements that can be achieved using these configurations. Under the assumption of perfect estimates of the applicable channel at the receiver, in a MIMO system with N transmit and N receive antenna elements, the received signal decomposes to N “spatially-multiplexed” independent channels. This results in an N-fold capacity increase relative to single-antenna systems. For a fixed overall transmitted power, the capacity offered by MIMOs scales linearly with the number of antenna elements. Specifically, it has been shown that with N transmit and N receive antennas an N-fold increase in the data rate over a single antenna system can be achieved without any increase in the total bandwidth or total transmit power. See, e.g., “On Limits of Wireless Communications in a Fading Environment When Using Multiple Antennas”, by G. J. Foschini et al, Wireless Personal Communications, Kluwer Academic Publishers, vol. 6, No. 3, pages 311-335, March 1998. In experimental MIMO systems predicated upon N-fold spatial multiplexing, more than N antennas are often deployed at a given transmitter or receiver. This is because each additional antenna adds to the diversity gain and antenna gain and interference suppression applicable to all N spatially-multiplexed signals. See, e.g., “Simplified processing for high spectral efficiency wireless communication employing multi-element arrays”, by G. J. Foschini, et al, IEEE Journal on Selected Areas in Communications, Volume: 17 Issue: 11, November 1999, pages 1841-1852.
Although increasing the number of transmit and/or receive antennas enhances various aspects of the performance of MIMO systems, the necessity of providing a separate RF chain for each transmit and receive antenna increases costs. Each RF chain is generally comprised of a low noise amplifier, filter, downconverter, and analog to digital to converter (A/D), with the latter three devices typically being responsible for most of the cost of the RF chain. In certain existing single-antenna wireless receivers, the single required RF chain may account for in excess of 30% of the receiver's total cost. It is thus apparent that as the number of transmit and receive antennas increases, overall system cost and power consumption may dramatically increase. It would therefore be desirable to provide a technique for utilizing relatively larger numbers of transmit/receive antennas without proportionately increasing system costs and power consumption.
The above-referenced copending non-provisional application Ser. No. 10/801,930 provides such a technique by describing a wireless communication system in which it is possible to use a smaller number of RF chains within a transmitter and/or receiver than the number of transmit/receiver antennas utilized. In the case of an exemplary receiver implementation, the signal provided by each of M (M>N) antennas is passed through a low noise amplifier and then split, weighted and combined in the RF domain with the signals from the other antennas of the receiver. This forms N RF output signals, which are then passed through N RF chains. The output signals produced by an A/D converter of each RF chain are then digitally processed to generate the N spatially-multiplexed output signals. By performing the requisite weighting and combining at RF using relatively inexpensive components, an N-fold spatially-multiplexed system having more than N receive antennas, but only N RF chains, can be realized at a cost similar to that of a system having N receive antennas. That is, receiver performance may be improved through use of additional antennas at relatively low cost. A similar technique can be used within exemplary transmitter implementations incorporating N RF chains and more than N transmit antennas.
The RF-based weighting techniques described in the above-referenced '930 non-provisional application advantageously enable the same type of combining of spatially weighted signals to be performed in the RF domain as is done at baseband. One advantage of these techniques is that RF weighting and combining may be performed using only N transmit and N receive RF chains, independent of the number of transmit and receive antennas. Furthermore, notwithstanding the fact that the '930 application describes RF-based weighting and combining, it remains possible to implement the digital signal processing schemes prior to conversion to analog/RF within the transmitter and subsequent to conversion to digital from analog/RF within the receiver. Such techniques may include successive interference cancellation in the case of MIMO systems (see, e.g., “V-BLAST: An architecture for realizing very high data rates over the rich-scattering wireless channel,” in Proceedings of URSI ISSSE, September, 1998, pp. 295-300).
Although the techniques described in the '930 non-provisional application may not offer performance identical to baseband techniques in the case of temporal and/or frequency domain signal processing, it may still be preferable to employ such techniques as a result of the lower costs involved. Frequency domain processing is used in systems in which, for example, the transmitted signal consists of a number of frequency subcarriers. This type of signal processing is required to be performed when implementing systems based upon orthogonal frequency division multiplexing (OFDM), such as the wireless local area network systems popularly referred to simply as “802.11(a)” and “802.11(g)”. Alternatively, for the same or lower cost as is required by conventional approaches, the techniques of the '930 application may be employed to enable the use of a greater number of antennas, which may result in substantially superior performance relative to such conventional approaches.
In the above-referenced copending non-provisional application Ser. No. 10/835,255, techniques for generating RF-based weighting values designed to maximize an output signal-to-noise ratio of the receiver averaged over the applicable channel were presented. Although this performance measure may be suitable for application in systems in which frequency domain processing involves the use of the same modulation and coding for each subcarrier (e.g., in 802.11(a) systems), in many cases overall data rate may be increased by varying the modulation and coding employed among the various subcarriers. When such frequency domain processing is performed at baseband, it is known that the output data rate may be maximized by adaptive bit loading; see, e.g., “Adaptive bit loading for wireless OFDM systems,” A. N. Barreto and S. Furrer, International Symposium on Personal, Indoor and Mobile Radio Communications, 2001, September/October 2001, pages G-88-92, vol. 2. Consistent with the adaptive bit loading technique, the power at which each subcarrier is transmitted is scaled based on the channel gain (while maintaining the aggregate transmitted power constant) and the data rate in each subcarrier is adjusted to the maximum value attainable in view of a given performance measure characterizing the receiver (e.g., bit error rate). The set of baseband spatial weights associated with each subcarrier that are used during baseband processing computations are calculated independently of the bit loading parameters using closed-form expressions. Once these baseband spatial weights have been determined, calculation of adaptive bit loading parameters applicable to each transmitted subcarrier is then separately performed. See, e.g., “On implementation of bit-loading algorithms for OFDM systems with multiple-input multiple-output”, J. Gao and M. Faulkner, Proceedings of the IEEE Vehicular Technology Conference, 2002-Fall, pages 199-203, vol. 1.
Unfortunately, methods of calculating spatial weighting values independently of the bit loading parameters using closed-form expressions of the type described above are inapplicable to the cases described in the above-referenced copending non-provisional patent applications; that is, in cases in which signal weighting is performed exclusively in the RF domain or within both the RF and baseband domains.